Porous solids with tailored pore characteristics have attracted considerable attention as selective membranes, photonic bandgap materials, and waveguides.[36, 37] Examples include porous membranes having highly ordered monolithic structures made of oxide materials,[38] and semiconductors.[35] Three-dimensionally porous metals have also been prepared from metals such as Au, Ag, W, Pt, Pd, Co, Ni and Zn,[10-14] formed in an inverse opal structure, where the metal is present in all the spaces between face center cubic (FCC) close packed spherical voids.
Metallic photonic crystals, metal based structures with periodicities on the scale of the wavelength of light, have attracted considerable attention due to the potential for new properties, including the possibility of a complete photonic band gap with reduced structural constraints compared to purely dielectric photonic crystals,[1] unique optical absorption and thermally stimulated emission behavior,[2, 3] and interesting plasmonic physics.[4] Photonic band gap materials exhibit a photonic band gap, analogous to a semiconductor's electronic band gap, that suppress propagation of certain frequencies of light, thereby offering photon localization or inhibition of spontaneous emissions. Photonic applications may include high efficiency light sources,[5] chemical detection,[6] and photovoltaic energy conversion.[3] Other applications include acoustic damping, high strength to weight structures, catalytic materials, and battery electrodes.[7] The photonic properties of metal inverse opal structures have been of significant interest because of the simplicity of fabrication and potential for large area structures. However, in practice, experiments on metal inverse opals have been inconclusive, [8-10] presumably because of structural inhomogeneities due to synthetic limitations.
A photonic band gap material, a three-dimensionally interconnected solid, exhibiting substantial periodicity on a micron scale has been fabricated using a colloidal crystal as a template, placing the template in an electrolytic solution, electrochemically forming a lattice material, e.g., a high refractive index material, on the colloidal crystal, and then removing the colloidal crystal particles to form the desired structure.[35] The electrodeposition provides a dense, uniform lattice, because formation of the lattice material begins near a conductive substrate and growth occurs substantially along a plane moving in a single direction normal to the conductive substrate.